Melt the butter with the canola oil in a Dutch oven over low heat. Add the onions and season with salt and pepper. Increase the heat to medium and cook the onions, stirring occasionally, until the onions are caramelized, 25-30 minutes. Add the garlic and sauté until fragrant, 1-2 minutes. Raise the heat to high and stir in the white wine. Once the liquid has reduced by half, pour the onion mixture into the slow cooker. Add the bay leaves, beef broth, and beef base. Cover and cook on high for 7 hours.
When the timer goes off, preheat the oven to broil. Arrange six ovenproof bowls on a baking sheet. With a slotted spoon, transfer the cooked onions to the bowls, being sure to discard the bay leaves. Ladle the hot soup into the bowls, leaving about 1/4" of space at the tops. Place 2 or 3 croutons on top of each bowl. Pile the Gruyère and sprinkle the Asiago on top of the croutons. Put the bowls under the broiler and broil until the cheeses melt and become bubbly and brown, 2-3 minutes. Serve the soup hot with additional croutons.
1 day-old baguette, cut diagonally into 1/2" slices
1/2 cup extra-virgin olive oil
1 Tbsp chopped fresh parsley
1/2 tsp garlic salt
1/2 tsp freshly ground black pepper
1/2 tsp onion powder
1 cup (4 oz) grated Asiago
Preheat the oven to 400°F.
Arrange the bread slices on a baking sheet. Whisk together the olive oil, parsley, garlic salt, pepper, and onion powder. Brush the oil-herb mixture onto the bread slices. Sprinkle the Asiago evenly over the croutons. Bake until lightly brown and the cheese is melted, 9–12 minutes. Watch the croutons carefully to make sure the cheese doesn't burn.
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